This invention relates to ballast circuits for HID lamps and, more particularly, to a ballast circuit which very accurately regulates the wattage of HID lamps, and particularly high-pressure sodium lamps, during prolonged operation thereof and which also limits the line current drawn by the lamp during starting to less than the line current drawn by the lamp during normal operation.
U.S. Pat. No. 3,873,910 dated Mar. 25, 1975 to Willis, Jr., discloses a variable inductor which includes a main winding and a control winding positioned on opposite sides of a gapped shunt. The control winding is adapted to be closed by a gate-actuated AC switch, and upon closing, the inductance of the variable inductor is decreased by a predetermined amount, thereby controlling the power input to the ballasted lamp.
In U.S. Pat. No. 4,037,148 dated July 19, 1977 to Owens, is disclosed a ballast device especially adapted to operate with a variable inductor as described in the foregoing U.S. Pat. No. 3,873,910 to ballast a high-pressure sodium discharge lamp wherein a non-linear amplifier is incorporated in circuit. For actual control, lamp voltage and line voltage are sensed and these voltage signals are combined in a programmable unijunction transistor to control the firing time thereof, and thus the actuation of the gate-controlled AC switch.
U.S. Pat. No. 3,886,405 dated May 27, 1975 to Kubo discloses sensing a variety of parameters in order to effect lamp control, including sensing both lamp voltage and line voltage to vary the impedance in circuit with the lamp and thus regulate lamp power consumption. In the case line voltage is sensed, the sensed voltage is applied to a unijunction transistor to control the firing time thereof, as in the afore-mentioned U.S. Pat. No. 4,037,148.
In U.S. Pat. No. 3,590,316 dated June 29, 1971, to Engel and Elms, the applicants herein, is disclosed a transistorized wattmeter which is used to control a variable impedance in order to control lamp wattage. The wattage is measured electronically and is converted into a current signal which is used to charge a ramp-type capacitor which in turn controls the firing of a gate-controlled AC switch when a predetermined voltage is achieved across the capacitor.
Lamp starter circuits for high-pressure sodium lamps are well known such as described in U.S. Pat. No. 4,072,878 dated Feb. 7, 1978 to J. C. Engel and G. F. Saletta.